This is now a concern for me also. I have the recommended Let'sGoAero bike rack, which is fine, but after adding a cover for the bikes (made by Swagman), it partially obscures the tail lights of my Ollie. Not too badly if someone is following me in an automobile, but sitting higher in a pickup or big rig would probably block my tail lights from their view. Of course eliminating the cover solves that problem, but it doesn't keep the bikes protected.
A rack at the front of the Ollie seems to be the best solution.
Doug